To add cost for addition item you will need to add the following variable to your button code.

“shipping2″

1. Login to your PayPal account.
2. Click Merchant Services tab at the top of the page.
3. Click the type of button you want. Example: Buy Now, Add to Cart, Donate, Subscribe
4. Enter your information to customize the button to your needs.
5. Click Step 3 for additional features.
At bottom of page in Advance Variable box include the following:

shipping2=4.95 (for an example of $4.95 in shipping-change to whatever the shipping should be)

6. Click “Create Button.”
7. Click “Select Code” to select the button code.
8. Copy and paste the button code into your own web page code.

And this will make the shipping extend to match the quantity